Tubular blown films have become very popular methods of producing inexpensive plastic bags and plastic sheets made from single or multiple layer plastic polymer materials, such as polyethylene, polypropylene, polyamide, ethylene vinyl acetate, ethylene vinyl alcohol and ionomers.
Typically, a tubular blown film is produced by extruding the plastic film from a ring-shaped extruder. In some cases, the blown film may contain multiple layers of different polymers co-extruded into one multi-layer film, having a total thickness and discernable internal layer thicknesses. Because blown films are tubular in shape, controlling the thickness profile of the tubular blown film or of its constituent layers (as well as the profiles of other physical properties of the tubular blown film) is often done using measurements made on the bubble using backscatter sensors such as NDC Gamma, Capacitance and optical sensors.
Measurement on the bubble is typically conducted by backscatter sensors which are relatively inaccurate, and cannot make truly reliable constituent layer measurements. It would be preferable to use transmission sensors which are more accurate than backscatter sensors and can in some versions also offer excellent constituent layer measurements as well as total measurements. However, transmission sensors cannot be fitted on the bubble as one sensor head would have to remain inside the bubble and this is impossible to achieve in practice.
In most blown film lines, the tubular blown film is folded (“collapsed”) after blowing, and measurements at each position on a collapsed film are possible using both backscatter and transmission technology. However, as two layers of film are always present at any given location, the problem of determining the measurement of the individual top and bottom layers must be solved.
Solution of this layer measurement problem would also give the possibility of a faster, more accurate measurements and provide the ability to accurately control the quality of the multilayer film.
Several prior art methods for controlling the thickness profile of the tubular blown film have attempted to control the thickness profile of the tubular blown film by using measurements made on the folded (double layer) film. The problem of measuring the folded blown film is that what is measured is the combined (double layer) thickness of two different sectors of the blown film. To effectively control the thickness profile of the blown film, it is necessary to determine the thickness of all (single layer) sectors around the circumference of the tubular blown film.
One prior art method of accomplishing this is to measure the combined thickness of the folded film only at the edges of the fold where the combined thickness of oppositely disposed sectors of the film can be assumed to be from the same sector. This method, however, requires fully rotating the blown film so that repeated measurements can be taken of each sector as each sector is rotated to the edges of the fold. This is time consuming, and can take 2 to 10 minutes, depending upon the speed of rotation of the blown film.
Another method of controlling the thickness profile of a tubular blown film is disclosed in European Patent EP 1 207 368 B1. In the method disclosed in this patent, the thickness of individual sectors of the blown film is estimated by a technique using the least squares method or weighted least squares method for solving hyper-constrained systems. The method described in this patent is also time consuming and requires about 10-20 scans of the folded film before thickness estimates of each sector can be calculated. Moreover, the resulting profile determination is merely an estimate of the true profile.
Accordingly, there is a need for a method for controlling the thickness profile of a tubular blown film which avoids the aforementioned problems in the prior art.